Influence of gravitational forces on function of lower extremity muscles

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Abstract
Action potentials were recorded by surface electrodes from the quadriceps, hamstrings, tibialis anterior, and triceps surae muscle groups as 12 normal adolescents performed a series of changes in posture. Postural movements consisted of sitting, standing, squatting, sit-ups, unilateral standing with contralateral lower extremity exercise, trunk forward bending with knee flexion and extension, and the head stand. Photographs were taken to assist in analyzing change in body position during the activities. The hamstrings and triceps surae resist forces tending to flex the supporting extremity when the trunk or contralateral limb is suspended anteriorly. The quadriceps and tibialis anterior act volitionally or by lengthening contraction to move the body vertically. The muscles participating in an activity and their specific functions are influenced by gravitational forces, the weight and position of various body segments, and the point of contact with the supporting surface. electromyography; thigh and leg muscles; postural exercises; human male and female Submitted on February 3, 1964